Dentifrice formulations comprising peroxide are known and useful for cleaning and whitening teeth. The peroxide can bleach the teeth, remove stains, and kill cariogenic bacteria. However, peroxide compounds are highly reactive, and consequently difficult to formulate. Moreover, hydrogen peroxide can spontaneously decompose to form oxygen gas (O2) and water, so that on storage, the dentifrice containers may bloat, burst or leak, and the remaining formulation will not have enough peroxide remaining to clean and whiten teeth effectively. Some initially comprise very high levels of peroxide, which decomposes over time, so that the exact amount of peroxide delivered on application is variable and largely depends on how long and under what conditions the dentifrice has been stored.
The peroxide may be present as hydrogen peroxide or as a source of bound hydrogen peroxide. Sources of bound hydrogen peroxide include PVP-H2O2 complexes, urea peroxide, calcium peroxide and sodium percarbonate.
Hydrogen peroxide and such sources of bound hydrogen peroxide are challenging to formulate into stable dentifrice products due to their reactivity with many common ingredients found in oral care products, especially water and abrasives. It is known in the art that peroxide stability can be improved by decreasing the formula pH. However, lowering the pH below 5.5, or even below an optimal minimum threshold of pH 6 when used in aqueous solution during brushing, in toothpaste designed to be used daily could lead to harmful erosion of enamel.
Single phase whitening dentifrice compositions are described, for example, in the Applicant's earlier WO-A-2012/102750, WO-A-2011/079167 and WO-A-2007/037961, and in US-A-2006/0062744, the contents of which are incorporated herein by reference. Those compositions are not acidic and comprise PVP-H2O2 complexes which stabilize the hydrogen peroxide in the composition, which has a low water content or is substantially anhydrous. By exposure to aqueous environments, as in the oral cavity, the PVP-H2O2 dissociates into individual species (PVP polymer and H2O2). The PVP-H2O2 complex is generally comprised of about 80% by weight polyvinyl pyrrolidone and 20% by weight H2O2. It has also been demonstrated in WO-A-2012/102750 that the careful selection of a calcium abrasive and an ethylene oxide, propylene oxide block co-polymer of a specific molecular weight can also be used to improve the stability of bound hydrogen peroxide in dentifrice.
However, known whitening dentifrice compositions including peroxide may exhibit an unacceptable level of peroxide decomposition and loss of whitening efficacy as a result of being stored prior to sale or by the user.
There is thus a need for improved peroxide-containing whitening oral care compositions, for example dentifrice compositions, which exhibit improved cosmetic stability of the peroxide, and so are chemically stable for long-term storage and are suitable for everyday consumer use without significant loss of whitening efficacy.
Furthermore, when formulating oral care compositions, for example dentifrice compositions, it is important that the formulation has physical stability and does not progressively separate into solid and liquid phases as a result of being stored prior to sale or by the user. Some known whitening dentifrice compositions including peroxide may exhibit a significant level of phase separation, which is unacceptable to the user.
There is thus a further need for improved peroxide-containing whitening oral care compositions, for example single phase dentifrice compositions, which exhibit improved cosmetic phase stability of the composition, and so are physically stable for long-term storage and are suitable for everyday consumer use without significant breakdown of the single phase.